5.1. RECENT DEVELOPMENTS
The IAEA CRP on LSNAA to which this document is dedicated was concluded in the year 2012. Several LSNAA facilities have been developed during the CRP, as described in Section 3, and very strong methodological advancements were achieved.
However, one further important outcome of the CRP was to bring about an increased awareness of the capability of NAA and PGA to measure chemical elements in large objects.
This has resulted in both new applications in existing facilities and in new facilities being established after the CRP ended. These will be described in this section.
An important next step in analysing large samples is the spatially resolved quantitative element measurement. Computer tomography and neutron radiography techniques already provide qualitative information about the distribution of elements. Combining this with e.g.
focused neutron beams and prompt gamma analysis opens a new unique area of applications.
The CRP participant from Japan published an attempt for simultaneous imaging and elemental composition measurement performed at the object, used for the laboratory intercomparison described in Section 2 [69]. Using a slit collimator (1 cm width, 7 cm height) they were able to detect inhomogeneities in the scanned layers of the vase for the elements H, B, Na and Si. Other groups are also working towards spatially resolved element measurement in large objects [70].
The absence of sample treatment makes LSNAA useful for bulk analysis of ultrapure materials for verification of specifications. In the past, this was to some extent already demonstrated by the analysis of low mass intact (thin) silicon wafers with dimensions up to 12.5 cm. Trace elements in the entire neutron activated silicon ingots (30 cm length, 5 cm diameter) were measured by Vins et al. [71], demonstrating the capability of measuring trace impurities even inside large crystals and not on the surface wafer layers only.
Menezes et al. applied the large sample technique to study the composition of dietary supplements [72]. Elias et al. measured impurities in the content of complete packages of commercially available dog food as collected from the shelf, which is a material composed of various granular components [73]. Yagob [74] demonstrated that LSNAA can be fitfully applied in dietary intake studies in which, on basis of the double portion approach, multi kilogram amounts of food is collected over 3-5 days. Analysing these amounts ‘as collected’
circumvents the homogenization and sub-sampling problems.
The projects in the frame of the IAEA CRP were mainly related to the application of LSNAA in research reactors. Several new applications have also been reported utilising D-T and D-D neutron generators. Ma and Mildenberger reported the development and use a method for measuring toxic elements in 200 L drums of radioactive waste, packed in concrete [75, 76]
using PGA and a 14 MeV n-generator. Yang [77], Naqvi [78] and Eftekhari Zadeh [79] used this combination for measuring trace elements in cement; Gierlik et.al. used the same approach for studies of explosives [80]. Monte Carlo based modelling of the interaction of the neutrons in the sample, the neutron and gamma ray self-attenuation and the voluminous photopeak efficiency have now become common [81].
belt-analysers using neutron sources are in fact based on large sample analysis. The same applies to in-vivo measurement of major components in (human, animal) bone and other tissues. Both techniques (belt-analysers and in-vivo NAA/PGA) are around for decades but remarkably enough, it has taken until the 1990s before these approaches were conceived as valuable complementary assets for, e.g., reactor-based NAA/PGA.
Although much less worldwide available, photon activation analysis shares many of the advantages of NAA and PGA for the analysis of large objects. A review of this technique has been published by Segebade et.al. [83] referring to examples of the analysis of large samples by photon activation. Stamatelatos et al. [84] demonstrated the use of photon activation analysis for multi-element measurements in intact large clumps (ca. 125 g) of metallurgical slag from a copper furnace; these measurements were done for an archaeological research project to gain more insight in the metallurgical techniques applied in the early bronze age.
The scope of applications may expand further. The domain of forensic science is also an area to explore. Objects can now be directly analysed ‘as collected’, circumventing all problems on the representativeness of a small sub-sample taken for analysis. Moreover, the object is not disturbed by sub-sampling or even destructed by dissolution, and is thus preserved as evidence.
5.2 OUTLOOK AND SCIENTIFIC CHALLENGES
Large sample neutron activation analysis can be considered to be still in its formative years, when compared to normal NAA, which has more than 80 years of history. From the current status and recent developments, as described in this report, several scientific challenges can be identified as outstanding:
As already mentioned in Section 3.2.6, neutron transmission measurement and CT scanning can provide valuable details as input for the neutron self-shielding and gamma ray self-attenuation. This will contribute to enhance the degree of trueness of the measured amounts of the elements;
Activation using epithermal and fast neutrons for inducing specific nuclear activation reactions will require research into the modelling of the self-moderation in large (inhomogeneous) samples;
Activation using isotopic neutron source arrays or D-D neutron generator arrays may be also an approach towards further development towards a field method of LSNAA.
The use of such sources implies attention to the change of the neutron energy distribution inside the sample as the much lower neutron fluence rate will have to be compensated with sample sizes that can easily reach 10 kg or more. Whereas belt analysers for major components are applied in many industrial areas, a stand-alone field method for also minor and trace element analysis may be of interest for, e.g., the screening of mine tailings, recycled electronic parts and other bulk materials, e.g. for compliance with import requirements;
Laboratory intercomparison studies with asymmetrically shaped objects, and objects with point or layered inhomogeneities for further underpinning the validation of the computational methods and demonstration of the degree of trueness of the results;
Representativeness studies in areas where a sample undergoes many treatment steps before a test portion is finally available. Analysis of the original material and of sub-samples taken after processing may provide unprecedented insight in sampling errors and sample handling errors;
A fundamental aspect that needs to be considered is the reporting of the measured data, i.e. as mass fractions or total amounts. Reporting in mass fractions indicates an assumption on the degree of homogeneity; possibly criteria may have to be developed for the acceptable homogeneity in large samples at which reporting in mass fractions is scientifically sound.